1. Field of the Invention
The present invention relates to a relief valve device, in an oil pump of, for instance, an engine for vehicles, that allows securing greater discharge pressure and flow rate at high revolutions of an engine in order to secure lubrication and cooling, while reducing discharge pressure and flow rate at low and medium revolutions of the engine in order to improve efficiency.
2. Description of the Related Art
Conventional relief valves that modify relief pressure depending on the magnitude of engine revolutions (high-low revolutions) have been disclosed (Japanese Patent Application Publication No. H5-26024) wherein a first relief pressure and a second relief pressure are set in accordance with the magnitude of engine revolutions (high-low revolutions). Japanese Patent Application Publication No. H5-26024 (FIG. 2) discloses a relief valve that has a solenoid valve 19, as well as a mechanical relief valve that comprises a valve body 15 and a piston 16 that resist, on account of oil pressure, the urging force of a coil spring 17.
The coil spring 17 is interposed, in a compressed state, between the valve body 15 and the piston 16, and the solenoid valve 19 controlled to be on/off according engine revolutions; as a result, the oil pressure to the piston 16, via a communication passage 18, is turned on/off, the piston 16 moves to the left or right, and there changes the mounting length of the coil spring 17. The magnitude of relief pressure by the mechanical relief valve changes also as a result.
In Japanese Patent Application Publication No. H5-26024, the mounting length of a spring is modified in order to cause a mechanical relief valve to operate in two stages. In order to modify the spring length, means is employed to supply oil pressure from both the left and right of the spring, such that switching between action of oil pressure from the left or from the right is accomplished through switching between oil passages in the solenoid valve. Oil passages must thus be provided both on the left and right of the mechanical relief valve in order to supply oil pressure from both the left and the right. The total length of the installed mechanical relief valve becomes thus longer.
The configuration of the mechanical relief valve includes not only the valve body 15 and the coil spring 17, but also, in addition, the piston 16, which results in an increased number of components. Also, this component must be accordingly worked into the layout (design), and thus the total length of the installed mechanical relief valve becomes proportionally longer in accordance with the layout.
The valve body 15 and the piston 16 are two components that can move by virtue of oil pressure. The presence of such two movable components makes it necessary to secure the operation and the dimensions of both components. This translates into complex structures and mechanisms, as well as a need for high-precision machining and assembly, while the operation becomes more difficult to guarantee. All these factors contribute to driving costs up.
When engine revolutions are high, oil pressure acts on the piston 16 at the rear face of the mechanical relief valve, the piston 16 is displaced to the left, and the mounting length of the spring is shortened; as a result, relief takes place at the second relief pressure, which is higher than the first relief pressure. Upon switching to action of oil pressure on the piston 16, the piston 16 at the rear face of the mechanical relief valve is displaced to the left in a state where oil pressure is acting on the valve body 15 at the front face. Therefore, oil pressure acts simultaneously from both sides, i.e. the front face and the rear face, onto the coil spring 17.